When php makes a verification code, why is the whole screen black and only the middle one is defined?

Share the experience of a verification code: when the verification code generated by php can't be displayed normally, first open the php script that generates the verification code in the browser and have a look. If some information similar to "picture error" is displayed, it means that your php program can't run normally. Then, you enter "header ("content-type: image/png "); Delete this sentence and refresh this page. You can find problems and solve them step by step according to the mistakes. Finally, no problem. Add "head" and have a look. "Title ("content-type: image/png ");" Tell the browser that "I" is a picture. With this sentence, the browser will not display php errors normally.